This invention relates to the field of high strength tension members, such as for sailboat rigging, and more particularly to continuous standing rigging for sailboats utilizing continuous tension members, particularly fiber composite members.
Sailboat designers and builders are constantly striving to improve the standing rigging that holds the masts in the generally vertical position. Since wind can exert tremendous force on the mast, spreaders and supporting rigging, the characteristics of the standing rigging are critical. Some aspects of standing rigging that can be improved include reducing weight, reducing elongation (stretch), reducing wind drag (windage) e.g., by reducing the diameter of the rigging and/or improving its aerodynamic qualities, and reducing the number of rigging parts. The use of high strength and lightweight composite fibers with or without a polymer matrix in lieu of metallic wire rope or metallic rod tensioning members can reduce the mast rigging weight and is important for improved sailboat performance since any weight reduction that takes place above the deck allows for a far greater reduction in the keel weight. Also, to the extent that the number of rigging lines and/or their profiles can be reduced or consolidated, windage can be further reduced.
To date, fiber composite standing rigging systems and rigging systems formed of other materials (e.g., twisted steel cable and solid metal bars) for sailboats have been largely designed and built to the general arrangement commonly known in the sailing industry as “discontinuous rigging”. Discontinuous rigging is defined as a standing rigging system supporting a sailboat mast or other similar structure that is made up of a number of discrete tension members. The number of discrete tension member elements in part depends on the number of stabilization strut members (typically called “spreaders” in the yachting field of application) required to support and hold the sailboat mast upright and generally straight. In a discontinuous sailboat mast and rigging assembly, each tensioning member, between any two attachment points, such as the deck, the mast and from spreader to spreader are discrete tensioning member elements with a terminal or end fitting at each end. Consequently, the number of terminal fittings and/or attachment point hardware elements (generally formed of metal) in the overall arrangement is high if there are multiple struts or spreaders required to adequately support the mast along its length. Thus, there is an attendant weight penalty for the large number of metallic terminal fittings in a discontinuous rigging configuration even though lightweight fiber composite tensioning members may be used. Additionally, the terminal fittings are necessarily large in size to accommodate the attachment scheme for connecting the discrete tension member to the mast, spreaders and each other, thereby increasing the wind drag of the entire system.
A typical discontinuous sailboat standing rigging configuration will have both vertical shroud tension members and diagonal shroud tension members. The vertical shroud members more or less extend in a vertical orientation and connect between the port and starboard sides of the deck area and the free ends of the spreaders. The uppermost extending vertical shroud member terminates as a cap shroud near the top of the mast. The diagonal shroud tension members extend from the deck area to the root end of the lowest spreader on the mast, and from ends of the spreader to the mast near the next spreader going up the mast. Consequently, there are numerous directional or angular changes along the length of the tension member general arrangement because the diagonal shroud tension members are necessary to hold the mast straight and because the spreaders are generally not of equal length.
In contrast with a discontinuous rigging system, any continuous rigging system will have numerous bends (directional or angular changes from a straight tension member line at every strut or spreader) along the length of the continuous tension members. These bends become potential areas of weakness in the tension member (e.g., twisted wire rope or a fiber composite tension member) unless the tension member can be tailored to have equal strain in operation across the tension member cross-section at any given point along its length. The weak points are created because the tension member does not have equivalent stress/strain capability across its full cross section in the area of the bend when under tension if the tension member is not made to the required shape including directional or angular changes. A tension member made in the straight form and bent into the required configuration necessary to create a continuous rigging system will have inherent weak points at every bend because the tension member material on the inside of the bend radius at every spreader will not have the same tensile stress capability as the outside radius at the bend in use. The typical point of failure will be at the outside radius of the bend where under a tensile load, the outer portion of the tension member has more strain than the inside of the bend portion and therefore can be overloaded beyond the tensile strength properties of the tension member material.
The ability to design and build truly lightweight and efficient rigging using the principles of continuous rigging versus discontinuous rigging for sailboats and other applications using metallic wire rope and/or metallic rods is currently limited. For example, it is possible to use a single metallic wire rope or metallic rod as a vertical shroud tension member for a conventional multiple spreader sailboat mast. In this example, the metallic tensioning member would be attached at deck level and could pass around various spreaders. In this example, no fittings are used at the various spreaders as would be typical for a discontinuous rig. The metallic rod would be bent around the various spreader ends along its length. There is some loss of strength where the metallic rod is bent but the yield properties of metal make this approach somewhat feasible. However, to date, there is no known way to taper the metallic rod tension member along its length thereby reducing weight aloft as the respective forces diminish. Furthermore, there is no known method to split the metallic rod or wire rope off to make other tension member elements creating necessary diagonal shroud members without additional tension member end fittings terminals. Thus, there is an incumbent weight and windage penalty. For this reason, discontinuous rigs have been the dominant practice for both metal and fiber composite rigging. It is possible to have multiple metallic tension members at various lengths (all anchored at deck level) and taper off the diameters of these tension members over the length of the mast (some members acting as diagonals) but this still is an inefficient design in terms of weight, complexity and windage. A continuous fiber composite rigging system would be a significant advancement in terms of optimized strength where required, limited stretch, and overall weight and windage reduction.
Accordingly, if the number of metallic terminals for the overall sailboat standing rigging system is significantly reduced, the result will be less weight, fewer parts, reduced wind resistance and improved strength characteristics.
There accordingly remains a need for improved sailboat rigging having a configuration specifically utilizing fiber composite materials, fewer metallic terminals, and a reduced profile.